1. Field of the Invention
The present invention relates generally to implantable defibrillators and relates more particularly to an improved configuration for a capacitor for use in an implantable defibrillator. The improved configuration permits more efficient packaging of the defibrillator components and a more physiologically desirable shape of the defibrillator housing.
2. Background Information
Implantable defibrillators are implanted in patients who can be identified as being likely to suffer cardiac arrhythmias such as ventricular fibrillation that can cause sudden death. The defibrillator detects the occurrence of ventricular fibrillation and automatically delivers defibrillating therapy. Implantable defibrillators in their most general form include appropriate electrical leads for collecting electrical signals generated by the heart, and for delivering electric shocks to the heart to provide defibrillation therapy. Also included are batteries, energy storage capacitors, and control circuitry connected to the leads, batteries and capacitors for sensing the electrical activity of the heart and for charging the capacitors and triggering the delivery of shocks through the leads. Implantable defibrillators can also include circuitry for providing cardioverting therapy for treating tachycardia, and for providing pacing therapy for treating bradycardia.
Defibrillation therapy generally involves rapid delivery of a relatively large amount of electrical energy to the heart at high voltage. Presently available batteries suitable for use in implantable defibrillators are not capable of delivering such energy levels directly. Consequently, it is customary to provide a high voltage storage capacitor that is charged from the battery via appropriate charging circuitry. To avoid wasting battery energy, the high voltage capacitor is not maintained in a state of charge, but rather is charged during an interval after the need for defibrillation therapy has been identified by the control circuitry, and immediately prior to delivering the shock.
The storage capacitors used in implantable defibrillators typically are required to deliver relatively large amounts of energy, on the order of 30 to 40 joules. With present implantable defibrillator capacitor technology, energy density does not exceed 2 joules per cubic centimeter. Thus, the capacitor is necessarily relatively large and constitutes a significant portion of the volume of the implantable defibriilator. Because of their large size, present implantable defibrillators usually must be implanted in the patient's abdomen.
It would be desirable to provide an implantable defibrillator that is smaller so that it can be implanted comfortably in the pectoral region, just as smaller implantable pacemakers are presently implanted. To accomplish this goal, it will be necessary to provide more efficient packaging of the components of the defibrillator within the defibrillator's housing. In the prior art, it has been proposed to replace the conventional cylindrical aluminum electrolytic "photoflash" capacitors with capacitors employing the same aluminum electrolytic technology, but configured to fit in a defibrillator housing having rounded contours. In particular, U.S. Pat. No. 5,131,388, issued Jul. 21, 1992, proposes an implantable defibrillator having a generally planar housing with curved edges that contains an aluminum electrolytic capacitor comprised of a planar layered structure of anode plates and cathode plates, wherein the plane of the anode and cathode plates is generally parallel to the plane of the housing. Such a configuration permits the capacitor to be shaped to conform to the curved edges of the housing and to partially surround other components, such as the batteries.
One consequence of such a layered structure is that the anode and cathode are each comprised of a multiplicity of individual plates which must be electrically connected in common via a multiplicity of individual electrical connections. In contrast, conventional cylindrical aluminum electrolytic capacitors are comprised of a continuous anode plate and a continuous cathode plate rolled spirally into a cylinder, with only a single electrical connection to each plate. I believe that the spiral construction is more easily manufactured because of the smaller number of electrical connections. I have therefore invented an improved configuration for a storage capacitor for an implantable defibrillator that permits more efficient packaging of the capacitor within the housing of the defibrillator and that permits a more physiologically advantageous shape for the defibrillator housing, while retaining the manufacturing advantages of more conventional capacitor construction.